The hematocrit value is the percentage of the volume of the blood that is occupied by red blood cells. During some medical procedures, such as, for example, autotransfusion during or after surgery, there is a need to increase the blood's hematocrit value. Increasing the blood's hematocrit value is currently performed in blood centrifuges where blood is introduced by a peristaltic pump.
A blood centrifuge substantially comprises two coaxial and rigidly coupled bell-shaped chambers arranged with one inside the other. The portion of space between the two chambers defines a cell that receives the blood. The cell is connected to the outside by an inlet tube and a discharge tube. The inlet tube and discharge tube are connected to the bell-shaped chambers by a rotary coupling. The blood centrifuge rotates the chambers about their axis while the tubes are kept motionless.
The centrifugation procedure entails a first step of filling the cell. The cell is filled by introducing blood through the inlet tube. The centrifugal force propels the blood away from the rotational axis. The blood centrifuge packs the red blood cells in the cell against the wall of the outer chamber. The red blood cells pack against the outer wall because they are more dense than the blood's other components. Other cellular components, such as white blood cells and platelets, are arranged in a thin layer known as buffy coat directly adjacent to the mass of packed red cells. The buffy coat assumes an orientation substantially parallel to the centrifuge's rotational axis. The separated plasma, the remaining component of blood, is arranged in a layer which lies above the buffy coat closer to the rotation axis. As filling continues, the buffy coat moves closer to the rotation axis displacing the separated plasma toward the discharge tube. When the plasma reaches the discharge tube the plasma flows out of the cell into an adapted collection bag. The outgoing flow of plasma continues until an optical sensor detects that the buffy coat has reached the discharge tube indicating the centrifuge is full. When the buffy coat reaches the discharge tube the filling step is complete. No additional blood is introduced into the centrifuge. The centrifuge now contains almost exclusively packed red cells and the buffy coat, since the separated plasma has been almost completely displaced from the cell.
Optionally, the filling step is followed by a washing step for the red blood cells and by an emptying step during which the cells are collected in a suitable bag. In any case, the invention relates to the filling step because the hematocrit value of the blood after filling remains substantially unchanged during the subsequent steps.
After the filling step, the hematocrit value of the collected blood is higher than the hematocrit value of the input blood. The hematocrit value of the collected blood varies with each centrifugation. The collected blood's hematocrit value depends on the trend of the input blood's hematocrit value over time, which is normally variable, and the flow rate of blood into the cell. For example, a low flow rate allows a high degree of packing of the red cells, with a high hematocrit value, but entails a long filling time which is sometimes incompatible with emergency conditions; or, alternatively, a high flow rate reduces the procedure time but the collected blood's hematocrit value is typically only slightly higher than the input blood's hematocrit value.
The flow rate of input blood is the only directly controllable variable for blood centrifugation during the filling step. Therefore, the flow rate is altered to adapt the collected blood to specific requirements. There is currently no system for automatically controlling the operation of a blood centrifuge. An operator typically controls the flow rate by adjusting the pump based on experience. The operator determines how the flow rate should be adjusted by continuously monitoring the centrifugation or by choosing among a certain number of predefined procedures, but these techniques have drawbacks. The drawbacks can include an inaccuracy in the result and considerable direct involvement of the operator. In any case, final hematocrit value and the time for centrifugation have never been predictable.